Walexan, Would you have considered using a Slimline "Pitts" style muffler instead of the stock one on your ST .45 ?. I just changed out my Enya .50 [ which I could not get to idle no matter what I did to it ] to a SuperTigre .51. It fit just fine with a Pitt's muffler and a lot less "hogging" out on the cowl. MM